This request is for a supplement to my current grant (HD-09328-01) to include a new requirement arising after submission of the original proposal. The basic study is designed to evaluate the interrelationships between dispersal and other population regulatory mechanisms of the prairie vole, Microtus ochrogaster. The effects of the following factors upon dispersal are being studied in the basic program: demographic changes in the population, fluctuations in the physical environment, changes in the quantity and quality of the vegetation, and variation in levels of aggressiveness. Correlation of qualitative differences between dispersers and nondispersers will aid in the identification of stimuli triggering dispersal, as well as evaluation of the significance of dispersal as a population regulatory mechanism. Owing to the immigration of a competing species, the meadow vole, Microtus pennsylvanicus, into the study region after the initiation of the research program, it is now essential that the program be expanded to analyze the effects of interspecific interactions on dispersal and, in turn, on population regulatory mechanisms. Comparisons will be made of rates of dispersal of given components of populations of the two species from an area and changes in aggressive behavior of each species. Analysis will be made of the reciprocal effects of changes in population density, demographic factors, and aggressive and dispersal behavior of the two species. A control study site, in which the meadow vole is eliminated, will be established to provide data necessary to separate intraspecific and interspecific factors affecting population regulatory mechanisms of the prairie vole.